Catalan separatist leaders accused of rebellion for trying to make their region independent from Spain launched their defense on Tuesday at the start of their long-awaited trial.

Sitting on benches in the ornate chamber of Madrid's Supreme Court, the defendants faced a row of judges and a Spanish flag in proceedings broadcast live on television. Twelve defendants are in the dock over an independence referendum, held on October 1, 2017 in defiance a court ban, and a short-lived declaration of independence, AFP reported.

Nine of them are charged with rebellion and three face lesser charges of disobedience and misuse of public funds.

The independence bid sparked Spain's deepest political crisis since the transition to democracy in the 1970s after the death of dictator Francisco Franco.

"This case targets political dissidence," said Andreu Van den Eynde, the lawyer of two defendants including Catalonia's former vice president Oriol Junqueras, who faces up to 25 years in jail.

The lawyer accused authorities of violating the defendants' fundamental rights.

In a politically-charged case, Spain has been forced to defend its judiciary against criticism of the proceedings against the Catalans.

Supreme Court President Carlos Lesmes alleged "a big smear campaign."

Many Spaniards support the proceedings, shocked by the actions of Catalonia's regional executive in October 2017.

Before the start of the trial, separatist officials demonstrated near the courthouse, holding a banner that read: "Deciding is not an offence".

More than 600 journalists were accredited to cover the trial.

Separatists in Catalonia want to hold a referendum on the region's future and have dismissed the trial as a politically-motivated "farce".

Pro-independence protesters in the region briefly blocked several roads before dawn, setting fire to tires and holding up traffic.

Carles Puigdemont, Catalonia's former president who fled to Belgium days after the independence declaration on October 27, is not among the defendants.

Spain does not try suspects in absentia for major offences.

Speaking from Berlin, Puigdemont said the trial was a "stress test for Spanish democracy".

He called on the international community for help, asking the European Union why it was more concerned with Venezuela than Madrid.

In his absence, the trial's main protagonist is his former deputy Junqueras, who opted to remain in Spain.

The 11 other defendants include members of Catalonia's former executive, the two leaders of powerful pro-independence associations, ANC and Omnium Cultural, and the former president of the Catalan parliament. They face jail terms of seven to 17 years.

The nine defendants charged with rebellion have been in pre-trial detention for months, some of them for more than a year.